The blades in a wind turbine are typically attached, at a blade root portion, to a rotor hub, either directly or through an extender.
The blades in a wind turbine are typically made from composite materials: reinforcing fibres may be bonded to one another with a resin and, subsequently, cured to consolidate. Different kinds of fibres (e.g. glass, carbon, combinations) and resins (e.g. epoxy) may be used in different forms (e.g. prepreg, fibre sheets). Such composite materials may not have the structural integrity to provide a secure fixing mechanism into which, for example, threaded bolts may be directly inserted. For this reason, it is known for wind turbine blades to comprise metal inserts or bushings which are embedded into the blade root in a direction parallel to that of the longitudinal axis of the blade. Studs, bolts or rods can be used in combination with these bushings to achieve a secure connection between the hub or extender and the blade.
A manufacturing process of a blade of a wind turbine in general terms may comprise a first step of molding and curing two separate halves of the blade made out of composite material and a second step of unmolding these two halves and joining them together. Alternatively, a complete blade may be manufactured in a single molding step. Once the blade has been manufactured it remains in storage until the wind turbine is finally installed. Typically, said storage may last from one month up to one year.
Although blades are made from composite material in order to reduce its weight, it is important to consider the fact that a finished blade, made of composite, still has a considerable weight.
Therefore, a problem derived from above mentioned storage time is that the blade root portion deforms in the storage position due to the weight of the whole blade (see FIG. 4).
FIG. 4 schematically shows a blade for a wind turbine in a storage position. Arrow A shows the force of the weight acting on the blade root portion 11 that may cause deformation of the mounting surface 111 of the blade root portion.
The orientation of the blade in FIG. 4 is merely one of several possible storage positions. The ideal storage position may depend on the length of the blade, its shape, its curvature and other design parameters of the blade.
Another problem, related to the one mentioned above derives from speeding up the manufacturing process of the blade. Blades or blade halves are often taken out of the mold when the curing process is not yet completely finished. The resin is thus still partly soft and the blade can thus more easily deform under its own weight.
In practice, the deformation of the blade root can lead to a more vulnerable attachment at the hub. The attachment holes of the blade may not be completely aligned with the attachment holes of the hub or extender, and the studs or bolts used for connecting the blade to the hub may not be completely centered in their holes. In operation, they thus may be subjected to transverse loads, potentially shortening their lifetime and the lifetime of the blades.